1. Technical Field
The present disclosure relates to light emitting diodes, and more particularly to driving circuits and their methods for driving light emitting diodes.
2. Related Art
Basically, the brightness level of light emitting diode (LED) changes with current and this characteristic is commonly applied.
A LED element works based on pulse width modulation (PWM). A switch circuit for switching the operation of a LED is connected to the LED, a reference current source and a PWM circuit respectively. If an ON period pulse signal is outputted by the PWM circuit, the switch circuit is at ON (i.e. being turned on), and the LED element will be driven and irradiated by a reference current supplied by the reference current source. If the period of ON level of the pulse signal is lengthened, the luminosity of the LED element will increase accordingly.
In order to reflect the grayscale levels and brightness level of an image, a duty cycle of PWM is changed by the PWM circuit to drive of the LED element, and the duty cycle is a proportion of conduction time of unit in an entire PWM period. In general, the PWM period can not be too long, e.g. can not be longer than 16.6 microseconds, in order to avoid the blinking of a screen from being perceived by human eyes. In order to obtain a wide dynamic range, the shorter duty cycle should be better, e.g. approximately 0.001 microseconds. However, the duty cycle is limited by how fast the circuit can drive.
In general, PWM mechanism is fulfilled by clock-based circuitry, the duty cycle can also be calculated by the designer using a pulse signal (CLK) time, in order to generate a PWM timing period. In order to expand the dynamic range of grayscale level, the signal of brightness level is converted into a binary data which is expressed in two to the Nth power and is designed using a plurality of units of length of time TCLK. In the case that N is set to be 16, and then the system is called a 16-bit PWM system. However, the bigger the N is in such a system, the longer is the PWM timing period, and the visual refresh rate becomes slower resulting in a blinking effect presented to human visual sensation. Furthermore, inconsistency will occur between the digital phase and the output of current in the switching between ON and OFF every time, i.e., switching error. The higher a frequency of ON and OFF, the bigger the switching error is.